/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 package com.crawler.utils

import java.io._
 import java.nio.channels.OverlappingFileLockException

 import org.slf4j.LoggerFactory

 /**
  * A file lock a la flock/funlock
  *
  * The given path will be created and opened if it doesn't exist.
  */
 class FileLock(val file: File) {
   val log = LoggerFactory.getLogger(this.getClass.getName)
   val channel = new RandomAccessFile(file, "rw").getChannel()
   private var flock: java.nio.channels.FileLock = null

   /**
    * Lock the file or throw an exception if the lock is already held
    */
   def lock() {
     this synchronized {
//       log.info("Acquiring lock on " + file.getAbsolutePath)
       flock = channel.lock()
     }
   }

   /**
    * Try to lock the file and return true if the locking succeeds
    */
   def tryLock(): Boolean = {
     this synchronized {
//       log.info("Acquiring lock on " + file.getAbsolutePath)
       try {
         // weirdly this method will return null if the lock is held by another
         // process, but will throw an exception if the lock is held by this process
         // so we have to handle both cases
         flock = channel.tryLock()
         flock != null
       } catch {
         case e: OverlappingFileLockException => false
       }
     }
   }

   /**
    * Unlock the lock if it is held
    */
   def unlock() {
     this synchronized {
//       log.info("Releasing lock on " + file.getAbsolutePath)
       if(flock != null)
         flock.release()
     }
   }

   /**
    * Destroy this lock, closing the associated FileChannel
    */
   def destroy() = {
     this synchronized {
       unlock()
       channel.close()
     }
   }
 }